Shrouded thrust bearings

ABSTRACT

A rotary blood pump includes a casing defining a pumping chamber. The pumping chamber has a blood inlet and a tangential blood outlet. One or more motor stators are provided outside of the pumping chamber. A rotatable impeller is within the pumping chamber and is adapted to cause blood entering the pumping chamber to move to the blood outlet. The impeller has one or more magnetic regions. The impeller is radially constrained in rotation by magnetic coupling to one or more motor stators and is axially constrained in rotation by one or more hydrodynamic thrust bearing surfaces on the impeller.

This application claims the benefit of U.S. Provisional ApplicationsNos. 60/758,793, filed Jan. 13, 2006; 60/758,892, filed Jan. 13, 2006;60/758,795, filed Jan. 13, 2006; and 60/758,794, filed Jan. 13, 2006,the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to rotary pumps and, more specifically, tocentrifugal rotary blood pumps and methods of therapeutic supportutilizing such pumps, in which an impeller within the pump rotates onwearless hydrodynamic and magnetic bearings which permit blood to bemoved from a pump inlet to a pump outlet by the impeller in contact onlywith the volume of blood within the pump.

BACKGROUND OF THE INVENTION

Clinical applications of ventricular assist devices to support patientswith end-stage heart disease, as a bridge to cardiac transplantation, oras an end stage therapeutic modality have become an accepted clinicalpractice in cardiovascular medicine. It is estimated that greater than35,000 persons suffering from end stage cardiac failure are candidatesfor cardiac support therapy.

Ventricular assist devices may utilize a blood pump for impartingmomentum to a patient's blood thereby driving the blood to a higherpressure. One example of a ventricular assist device is a LeftVentricular Assist Device (LVAD). The LVAD is attached to the leftventricle of the patient's heart where oxygenated blood enters the LVADthrough a blood inlet of the LVAD. The LVAD then imparts momentum to theblood. By connecting a blood outlet of the LVAD to the patient's aorta,pumped blood may reenter the patient's circulatory system.

Ventricular assist devices, such as the LVAD, have heretofore utilizedpositive displacement pumps and rotary pumps. Positive displacementpumps force blood from a first chamber to a second chamber by reducingthe volume of the first chamber while increasing the volume of thesecond chamber to draw blood into the chamber. Such pumps are normallyprovided with check valves that only permit flow in one direction andare normally large and prone to mechanical wear. The human heart is anatural example of a positive displacement pump. A rotary pump forcesblood by the spinning of an impeller within the pump. Known types ofpumps utilize an impeller to impart momentum to the blood through theuse of propeller type impeller blades which push the blood.

Rotary blood pumps may be either centrifugal or axial. In a centrifugalblood pump, blood enters the pump along its axis of rotation and exitsthe pump perpendicular to the axis of rotation. In an axial blood pump,blood enters the pump along its axis of rotation and exits the pumpalong the axis of rotation.

Traditionally, rotary blood pumps include a rotor consisting of a shaftand an impeller coupled to the shaft. Mechanical bearings are used tostabilize the rotor, both axially and radially, so the impeller couldremain free to rotate smoothly while being constrained in the axial andradial directions. Mechanical bearings within the volume of blood havebecome the source of thrombosis. Moreover, as the use of mechanicalbearings necessitated the protrusion of the shaft beyond the pumpingchamber, a seal was required to prevent the escape of blood from thepumping chamber. This too became a source of thrombosis and sometimeshemolysis, as well as premature wear.

The use of seals for mechanical shafts in rotary blood pumps has beenshown to be suboptimal as seals could cause thrombosis of the blood andcould wear out prematurely. To minimize the risk of thrombosis andfailed seals, sealless rotary blood pumps have been developed. Forexample, U.S. Pat. No. 5,695,471 to Wampler and U.S. Pat. No. 6,846,168to Davis et al. (the '168 Patent), both herein incorporated byreference, relate to sealless rotary blood pumps. In such seallessrotary blood pumps, the rotor and/or impeller may be suspended withinthe pumping chamber by the use of magnetic and/or fluid forces.

Magnetic and/or fluid forces used to suspend the impeller within thepumping chamber could serve to stabilize the impeller, allowing forrotation while preventing excessive axial or radial movement. Wearlessstabilization of an impeller can be achieved by magnetic bearings andhydrodynamic bearings. In this way, magnetic forces form magneticbearings and fluid forces form hydrodynamic bearings.

Several forms of magnetic bearings have been developed. In one form,passive magnetic bearings in the form of permanent magnets can beembedded in both the rotor and the pump housing to provide magneticcoupling that may keep the impeller suspended in position within thepump casing. Such permanent magnets embedded in both the rotor and thepump casing provide repulsive forces that may keep the impellersuspended within the pump casing. Such magnetic bearings are said to bepassive magnetic bearings as no control is used to keep the impellerproperly centered. While passive magnetic bearings may be effective atkeeping the impeller suspended in one direction, for example in theradial direction, it has been shown that such passive magnetic bearingsalone cannot keep an impeller suspended in both the axial and radialdirections.

Active magnetic bearings in the form of electromagnets can be used, forexample in or on the pump housing, magnetically to couple with and todrive the impeller. Power to the electromagnets may then be varied, asrequired, to adjust the magnetic field in response to displacement sothat the impeller may be kept in position.

Electromagnets may also be used, for example, in the pump casing, toprovide the repulsive magnetic force. These bearings are said to beactive magnetic bearings as the magnetic fields are actively controlledto maintain proper impeller position.

Because of the complexity of active magnetic bearings, rotary bloodpumps have been developed to use both passive magnetic bearings andhydrodynamic bearings to suspend the impeller in a sealless rotary bloodpump. For example, U.S. Pat. No. 6,234,772, to Wampler et al. (the '772Patent), herein incorporated by reference, relates to a sealless rotaryblood pump with passive magnetic bearings and hydrodynamic bearings. Inthe '772 Patent, radial suspension is enabled by a series of magneticdiscs within the impeller shaft and corresponding series of magneticrings in the pump casing. In the '168 Patent, radial suspension isenabled by a series of magnetic rings within a spindle that protrudesthrough a hole in the center of the impeller. A corresponding series ofmagnetic discs is provided within the impeller whereby the impeller issuspended about the spindle during rotation. In the '772 Patent, axialsuspension is enabled by a set of hydrodynamic thrust bearing surfaceson the impeller.

There remains a need for smaller and more efficient rotary blood pumps.In particular, there remains a need for wearless centrifugal pumps withhydrodynamic bearings and improved continuous fluid flow paths withinthe pump to further diminish the risks of hemolysis and thrombosis inthe blood being pumped. By developing more sophisticated rotary bloodpump impellers with hydrodynamic bearings and passive magnetic bearings,the physical size, performance and efficiency of the rotary blood pumpmay be improved to the point where consistent and reliable therapeuticsupport may be provided.

BRIEF SUMMARY OF THE INVENTION

A centrifugal rotary blood pump for implantation within the pericardialspace includes a housing defining a pumping chamber. The pumping chamberhas an axial blood inlet and a tangential volute defining a bloodoutlet. One or more magnetic motor stators are provided outside of thepumping chamber. A rotatable impeller is within the pumping chamber andis adapted to pressurize blood entering the pumping chamber for exitingat the blood outlet. The impeller has one or more magnetic regions. Theimpeller is radially and axially suspended in rotation by magneticforces created by passive and active sources of magnetic flux actingupon the impeller and one or more hydrodynamic thrust bearings providedon an upper surface of the impeller. The housing assembly may have anupper or front casing and a rear or lower casing which, when assembled,form a substantially cylindrical pumping chamber and a volute having atangential blood outflow port. In one embodiment, when assembled, thehousing defines a substantially cylindrical pumping chamber. Arelatively short inflow cannula is integrated with the upper casing andis adapted for insertion into a ventricle of the heart. The outflow portis directed perpendicular to the axis of the inflow cannula. The bloodinflow cannula may be straight, curved or otherwise bent to facilitatethe fit of the blood pump into the thoracic cavity of the patient or toimprove blood flow characteristics.

An electromagnetic motor for driving the pump consists of fixedelectromagnetic stator portions outside the blood flow region and theadjacent rotatable impeller within the pumping chamber adapted to createfluid pressure within the pumping chamber so that blood moves from theinflow to the outflow port. In one embodiment, the motor is a dualstator axial flux gap design with the impeller located within thepumping chamber between spaced apart motor stators. An upper motorstator is located adjacent or on the upper or front casing and a lowermotor stator is located adjacent the lower or rear casing. Each motorstator contains a plurality of electrical coils or windings arranged ona substantially circular iron core member for efficient electromagneticcoupling with corresponding magnetic regions of the impeller to causethe impeller to rotate within the pumping chamber. The upper motorstator may be positioned closer to the impeller than the lower motorstator to impose an axial magnetic preload on the impeller to counterthe magnetic impact on the impeller of the lower motor stator. In somesituations a single stator is placed on or adjacent the upper casing forthe same purpose. In one embodiment, each motor stator is co-axial withthe rotational axis of the impeller. The impeller and each motor statorare essentially circular in horizontal cross section and may havesubstantially the same diameter to aid in radial stiffness of therotating impeller during operation of the pump. Electrical power isdelivered to the coil windings by a plurality of power cables carriedwithin an elongated pliable cylinder. In one embodiment the pliablecylinder is made from silicone and may have a urethane sheath. Thepliable cylinder has a plurality of lumens therein, each of whichcarries a power cable. In one embodiment there are six such lumens.

The impeller has a substantially circular circumference and may beformed from a ferromagnetic substance. Ferromagnetic substances may bematerials that are strictly ferromagnetic as well as materials that areferrimagnetic. A suitable ferromagnetic substance may be, for example,compression bonded neodymium or Alnico (aluminum-nickel alloy). Aferromagnetic impeller allows for the magnetization of various regionsof the impeller in a desired configuration. A ferromagnetic impeller maybe treated with a conformal, protective polymer coating of an organicpolymer such as Parylene, or silicone, to prevent oxidation by forming ahermetic seal around the rotor. On top of this, a hard, lubriciousprotective coating may be applied over the conformal polymer coating, toprotect against wear and abrasion. Such coatings may include chromiumnitride, titanium-nitride, or other commercially available coatings suchas ME92, Med Co 2000, or DLC. A suitable ferromagnetic substance isbiocompatible, for example, a platinum-cobalt alloy may be used. Wherethe magnet material is biocompatible, the impeller need not be coatedwith a biocompatible material. In one embodiment, the impeller consistsof a plurality of raised solid or hollow bodies having a combination ofplane and curved side-wall surfaces, the bodies being spaced apartaround the impeller periphery. The outer peripheral side wall of each ofthe bodies is convex in the radial direction with a radius of curvaturethat corresponds to the overall circular circumference of the impeller.The plane surfaces are flat, and two straight side walls are of unequallength. The side walls of unequal length extend inwardly from the convexperipheral side wall of the body to intersect at angle of approximately90 degrees. The impeller bodies are similarly shaped. In each case theirvolume increases from the point of intersection of the two straight sidewalls to their convex peripheral side wall. The impeller is centrallyopen thereby defining an axial blood flow passage to the bottom wall ofthe pumping chamber. The intersecting side walls of the impeller bodiesare rounded to minimize thrombosis and hemolysis. The impeller bodiesare spaced apart by fluid flow paths therebetween that are defined bythe sidewalls of the raised bodies. The impeller bodies may bemagnetized to interact with magnetic forces imposed by the motor statorsto permit the impeller to be rotated within the pumping chamber. Theimpeller is magnetically and hydrodynamically suspended from contactwith the pump housing both radially and axially when the pump isoperating. Hydrodynamic axial thrust forces acting in one direction arecreated during operation of the pump by at least one inclined or taperedsurface area formed on an upper projection surface of at least one ofthe raised bodies adjacent to an internal surface of the upper pumpcasing. In some embodiments one of such bearing surfaces may be formedon each of the upper projection surfaces such that a plurality of suchtapered surface areas may be utilized, as desired. Each such taperedsurface area defines a hydrodynamic bearing surface. As the impellerrotates, blood engages the bearing surfaces at a relatively low pressureleading end of the bearing surface and is compressed against theinternal surface of the upper pump casing by the inclined bearingsurface which thereby creates a higher pressure exit or trailing end,causing an increase in fluid pressure acting axially on the impeller.Shrouds may be formed on the inner and outer sides of the taperedsurface area to prevent fluid leakage. A pressure relief surface may beformed on the impeller downstream of and adjacent the exit end of eachinclined bearing surface. The pressure relief surface is tapered todiverge from the inclined bearing surface thereby forming an area oflower fluid pressure to permit the blood to be directed into one of theseveral fluid flow paths between the raised bodies of the impeller. Thebottom of the impeller is covered by a substantially flat, smooth diskparallel to the bottom wall of the pumping chamber. Each flow pathbetween adjacent impeller bodies is substantially uniform incircumferential width. The longer side wall of one impeller body facesthe shorter side wall of an adjacent impeller body across and defining afluid flow path therebetween. The longer and shorter side walls definethe sides of each of the fluid flow paths. In this embodiment, thelongitudinal axis of each flow path defines an angle with thelongitudinal axis of each of the flow paths adjacent to it on eitherside of approximately 90 degrees.

Alternatively, the impeller bodies may be formed as hollow titaniumcasings. Each such casing defines an interior cavity which may be fittedwith a permanent magnet. Each inserted magnet is held within itsassociated cavity by a cap element or by a circular disk that covers thebottom of the impeller. In either case, the cap or disk is hermeticallysealed to the casing, such as by laser welding. Solid walls between thehollow casings may contain a plurality of bores to modify the weight ofthe impeller and to provide consistent rotation. A passive magneticbearing provides radial impeller support for rotation of the impelleraround a center post within the housing without contact with the postduring operation of the pump. In one embodiment, the magnetic bearingfor the impeller is created by repulsive forces of magnetic vectorsprovided by corresponding permanent magnets. Magnetic vectors created byone or more such permanent magnets located within the impeller areadapted to repel magnetic vectors resulting from one or more permanentmagnets located within the center post around which the impeller rotateswithout contact during pump operation. Such an arrangement providesradial stiffness for the rotating impeller and leaves an open spacebetween the impeller and the center post which defines a portion ofanother of several fluid flow paths through the impeller.

In one embodiment, the axial alignment between the magnets within theimpeller and the magnets within the center post is adjustable to providerepulsive magnetic preload forces acting on the impeller in an axialdirection opposite to the axial forces imposed on the impeller as aresult of hydrodynamic thrust. The magnetic preload enables the impellerto avoid contact between its bottom surface and an interior surface ofthe lower pump casing. This ensures yet another blood flow path aroundthe impeller which enables fluid pressure within the pumping chamber tokeep blood below the impeller in motion since blood is moved frombeneath the impeller up through the annular space between the impellerand the center post around which it rotates. The magnetic preload mayalso be sufficient to restore the impeller to its original positionshould it undergo a significant shock event. Motor electromagneticforces may also provide supplemental axial magnetic preload as well assupplemental radial impeller support. Magnetic preload enables theimpeller to avoid contact between its bottom surfaces and the lowerinterior surface of the lower pump housing casing. During operation, theaxial force produced by hydrodynamic thrust bearing surfaces on theupper projection surfaces of the impeller bodies moves the impeller awayfrom the upper wall of the housing but permits a blood flow path betweenthe lower projection surfaces of the impeller and the lower wall of thehousing. Fluid pressure within the pumping chamber keeps blood in motionbelow the impeller. Blood may move from beneath the impeller up throughthe open center of the impeller as the impeller rotates.

In one embodiment, the motor stators are concentric with the impellerand have substantially the same diameter such that magnetic interactionbetween the motor stators and magnetic regions of the impeller assistsin creating radial impeller stiffness. Axial preload on the impeller mayalso be provided by locating a motor stator on the upper pump housingcasing in close proximity to the impeller. In a dual motor statorembodiment, axial preload on the impeller may be provided by locatingthe upper motor stator closer to the impeller than the lower motorstator. As a result of balanced forces acting in axially oppositedirections on the impeller and the unique structure of the impeller, theimpeller is effectively dynamically suspended between the upper andlower casings of the pump housing during operation of the pump. Blood isthereby forced to move about the impeller and through the pumpingchamber without hemolysis or thrombosis. It will be understood thatmagnetic forces may be provided by permanent magnets, by electromagneticcircuits or by a combination of both such sources of magnetic forces. Asa result of preload and hydrodynamic forces acting in axially oppositedirections on the impeller and the unique structure of the impeller, theimpeller is effectively dynamically suspended between the upper andlower casings of the pump housing during operation of the pump. Blood isthereby forced to move about the impeller and through the pumpingchamber without hemolysis or thrombosis. It will be understood thatmagnetic forces may be provided by permanent magnets, by electromagneticcircuits, by magnetization processes or by a combination of such sourcesof a magnetic flux field.

The method of operation includes apical implantation of a short inflowcannula into the left ventricle of a heart, pressurizing the inflowingblood fluid within a pumping chamber by causing rotation therein of animpeller without mechanical contact with the impeller, positioning therotating impeller to be suspended within the chamber so as to becompletely submerged in the inflowing blood fluid, causing the inflowingblood fluid to traverse at least three flow paths within and around theimpeller whereby pressure within the pumping chamber causes continuousflow of the blood from the inflow to an outflow from the pumpingchamber, and directing the outflowing blood through a tube graft to theaorta.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the present invention, reference may behad to the accompanying drawings from which the nature and attendantadvantages of the invention will be readily understood, and in which:

FIG. 1 is an exploded view of a rotary blood pump according to anembodiment of the present invention;

FIG. 2 is a perspective view of the outer surface areas of an impelleraccording to an embodiment of the present invention;

FIG. 3 is a perspective view of a section of the outer surface area ofthe impeller of FIG. 2 which contains a hydrodynamic bearing surface;

FIG. 4 is a perspective view of the underside of the impeller of FIG. 2;

FIG. 5 is a cross-sectional view of an assembled rotary blood pumpaccording to an embodiment of the present invention;

FIG. 6 is a sectional view of a portion of a passive magnetic bearingstructure for an impeller according to an embodiment of the presentinvention;

FIG. 7 is an exploded view of a magnetic assembly for supporting anddriving an impeller according to an embodiment of the present invention;

FIG. 8 is a top plan view of a motor stator according to an embodimentof the present invention; and

FIG. 9 is a system view of an implanted rotary blood pump according toan embodiment of the present invention.

FIG. 10 is a top plan view of a rotary blood pump according to anembodiment of the present invention to which a pliable cylinder carryingpower cables is attached.

FIG. 11 is a cross section of the pliable cylinder of the presentinvention illustrating a plurality of lumen therein for carrying powercables.

DETAILED DESCRIPTION OF THE INVENTION

In describing the embodiments of the present invention illustrated inthe drawings, specific terminology is employed for sake of clarity.However, the present disclosure is not intended to be limited to thespecific terminology so selected, and it is to be understood that eachspecific element includes all technical equivalents which operate in asimilar manner.

Referring now to FIG. 1 there is shown a rotary blood pump 10 having apump housing that consists of a substantially circular front or upperpump casing 1 and a substantially circular rear or lower pump casing 2of equal diameter that interlocks with the upper pump casing 1 to form aclosed pumping chamber between them. The configuration of the upper andlower pump casings is such that the assembled pump housing defines asubstantially cylindrical pumping chamber 3 therein (FIG. 5). In oneembodiment the pumping chamber has a displaced volume of 45 cc. Theupper pump casing 1 may have a plurality of peripheral positioning holes4 for receiving a corresponding plurality of positioning pins 6projecting from the periphery of the lower pump casing 2. Theconfiguration of positioning holes 4 and positioning pins 6 ensures thatthe upper pump casing 1 and the lower pump casing 2 interlock in thecorrect position when the rotary blood pump 10 is assembled. The contactarea between the upper pump casing 1 and the lower pump casing 2 may besealed, for example using screws or a chemical sealant.

In the embodiment shown in FIG. 1 blood is supplied to the pump throughan axial inlet cannula 7 adapted for apical insertion into a heartventricle. The cannula 7 is affixed to or may be integral with the upperpump casing and is in fluid flow communication with the pumping chamber3 of the pump 10. As shown in the cross sectional view of FIG. 5, anembodiment of the inflow cannula 7 is of two-piece design, consisting ofan outer cylindrical section 8 and a coaxial inner cylindrical section9. The outer cylindrical section 8 of the inflow cannula 7 maybe weldedin an appropriate sealable manner to the outer surface of the upper pumpcasing 1. The inner cylindrical section 9 defines an inlet channel 11for the blood when the pump is installed and in operation. The sections8 and 9 may be laser welded together at the outer end 12 of the cannula,shown in FIG. 1. In one embodiment, the outside diameter of the outersection 8 is about 0.81 inches while the inside diameter of the innersection 9 is about 0.50 inches.

In one embodiment, the pumping chamber is in fluid-flow communicationwith a volute or diffuser section to avoid alteration of the position ofthe impeller in a radial direction as blood pressure increases duringoperation of the pump. The upper pump casing 1 and lower pump casing 2together define the diffuser by a pair of complementary upper and lowerhalf-round sections 14 and 16 formed as part of the upper and lowerhousing casings, respectively. The sections 14 and 16 together define ashort open-ended cylindrical diffuser tube. The diffuser extendscompletely around the circumference of the pump terminating at atangential outlet port 13 (FIG. 5). In one embodiment, the cross sectionof the diffuser section enlarges from an inlet end along its length to amaximum at the outlet 13. Blood exits the pumping chamber 3 through theoutlet 13 in a direction substantially perpendicular to the longitudinalaxis of the inlet cannula 7, an arrangement that has been found to beanatomically advantageous for locating the pump in the pericardialspace. When the pump is installed and in operation, the outlet 13 isadapted to be joined to an outflow graft 17, shown in FIG. 9, which inturn is suitably connected to the aorta 18. In one embodiment, the pumphousings or casings and the cannula may be made of titanium, abiocompatible titanium alloy, or a biocompatible ceramic material. Thepump structure may be machined from titanium or an alloy thereof.Alternatively, the pump structure, including the cannula, may be formedentirely from ceramic material.

Sealing of the cannula 7 to the heart ventricle may be accomplished withthe assistance of a peripheral ring groove 19 (FIG. 5) formed in theouter cylindrical surface of the cannula near the upper pump casing 1.The ring groove is fitted with an annular O-ring to provide a leak proofseal to a sewing ring of a ventricular connector [not shown] of the typedescribed, by way of example, in commonly owned U.S. Pat. No. 6,732,501.According to another embodiment, a peripheral ring groove is unnecessaryand an O-ring surrounding the cannula may be incorporated into thesewing ring to ensure a leak proof seal.

With reference to FIG. 1, a motor rotor or pump impeller 22 is locatedwithin the pumping chamber 3 between the upper pump casing 1 and thelower pump casing 2. The impeller 22 is circular in cross section andmay have a diameter of an inch or an inch and a quarter. The impeller isprovided with a central hole 23. A center post or spindle 24 is attachedto the lower pump casing 2 and protrudes from the axial center thereofthrough the impeller hole 23 when the pump is assembled to supportrotation of the impeller in the manner described in detail below. Thecenter post 24 is provided with a peripheral lower flange 26 by which alower annular ceramic disc 27 is retained to an interior surface of thelower pump casing 2. In one embodiment, the gap between the outerdiameter of the center post 24 and the diameter of the impeller hole 23is in the range of from 0.019 inches to 0.029 inches. The top portion ofthe center post 24 is formed as a conical surface 28. A substantialportion of the conical surface 28 of the center post protrudes above theimpeller hole 23 during operation of the pump. In one embodiment, theradius of curvature of the cone shape is a relatively constant 0.389inches. The tip of the cone is not necessarily a sharp point having, inone embodiment, a blending radius of 0.010 inches.

In operation, blood entering the cannula 7 from a heart ventricle passesaxially over the conical surface of the center post 24 into the pumpingchamber 3 where it is engaged by the rotating impeller. Blood enteringthe pumping chamber from the cannula 7 is redirected from axial flowexiting the cannula to a radial flow within which the impeller 22 issubmerged. The rotating impeller presses the blood radially into awhirling motion as a result of the configuration of the spinningimpeller, described in detail below, and moves within the diffuser atthe perimeter of the pumping chamber to the outlet 13.

The upper pump casing 1 may contain the upper half 29 of an electricalfeed through connector and header for a power and control cable tosupply power to the electrical motor of the pump. The lower pump casing2 may contain a corresponding lower half 31 of the electrical header.When the pump is assembled, the upper and lower halves 29 and 31interlock to form the header through which feed-through power wires areconnected to the electromagnetic motor stators. In one embodiment, thefeed-through wires are platinum. A PEEK header may be used to connectthe feed through wires to the external drive cables. The header may bemade of a material such as PEEK or a suitable plastic such as Tecothanor polysulfone. The header may also be made of a medical grade epoxy.With reference to FIG. 10, the upper half 29 of a PEEK front pump headeris shown connected to the power line cable through a strain reliefsection 81. The strain relief section is, in turn, connected to apliable elongated cylinder 82 which may be as long as desired to reach asuitable external power source, which may be the output of a controller(not shown). A connector and locking plug device 83 for connection tothe power source is affixed to the tubing 82 at its distal end. Thepliable cylinder is adapted to carry a plurality of power cables tocarry electrical power to the pump. In one embodiment the pliablecylinder is made of silicone. The pliable cylinder may be covered by athin urethane sheath (not shown) for extra abrasion resistance. It willbe understood by those skilled in the art that other bio-compatiblematerials such as urethane may be used for the pliable cylinder withoutdeparting from the scope of the present invention. Referring to FIG. 11,the pliable cylinder 82 contains a plurality of lumens 84 havingcircular cross sections, through each of which individual power cablesare threaded. In one embodiment there are six such lumens spaced aroundthe center of the cylinder adjacent its periphery in a generallycircular configuration. The centers of each lumen are approximately 60°apart. In one embodiment, the diameter of the pliable cylinder is about0.138 inches and the diameter of each lumen is approximately about 0.034inches. Such lumens can be used to hold power cables having a diameterof about 0.029 inches. The use of individual lumens within the cabletubing 82 has the benefit of extra fatigue resistance because theindividual power cables cannot rub together. In addition, whenreplacement is necessary replacement in situ is enabled since one powercable at a time may be replaced in order to minimize any stop time ofthe pump. A strain relief mechanism 86 may be used adjacent the distalconnector and locking plug device.

Referring now to FIG. 2 the impeller 22 is shown in greater detail. Inthis embodiment, the impeller is substantially circular in cross sectionand has a plurality of identical substantially hollow raised bodies 32circumferentially arranged thereon. Each of the raised impeller bodies32 has a generally right triangular cross section in a horizontal plane,with a curved hypotenuse defining a portion of the circumference of theimpeller. In one embodiment there are four such raised impeller bodies,the mid points of which are approximately 90 degrees apart.

The raised impeller bodies 32 are separated by flow slots or channels 33adapted to permit the flow of blood from the central portion of theimpeller to the surrounding pumping chamber. In one embodiment, thewidth of each of the slots 33 is about 0.150 inches. The flow slots 33are defined by vertical planar sidewalls 33 a and 33 b of unequal lengthextending parallel to but offset from a diameter of the impeller. In oneembodiment, the sidewall closest to the diameter of the impeller, forexample the sidewall 33 a of FIG. 2, is offset from the diameter byabout 0.164 inches. Each of the slots 33 has a downward-sloping bottomsurface 33 c, which constitutes an inclined ramp forming an angle ofabout 32 degrees with the horizontal. The exit points of the flow slots33 at the circumference of the impeller are approximately 90 degreesapart. Each ramp surface 33 c is longitudinally at right angles with thecorresponding longitudinal axis of the flow slots on either side.

The primary flow path for blood entering the inflow cannula 7 is tostrike the conical surface 28 of the center post 24 and pass through theflow slots or channels 33 to fill the pumping chamber. As indicated, therotating impeller causes the fluid pressure in the pumping chamber toincrease resulting in continuous movement of the blood from the inflow11 to the outflow port 13.

The upper surface of each impeller block 32 is provided with a curvedand tapered or inclined ramp 34 defining an axial hydrodynamic bearingsurface. In one embodiment, each ramp surface 34 spirals upward in aclockwise direction from a relatively lower fluid pressure entranceregion 36 to a relatively higher fluid pressure exit region 37. Theangle of inclination of the bearing surface 34 is less than one degreerelative to the horizontal. When the impeller 22 is rotating, thesidewalls 33 a define leading edges so that blood passing over thehydrodynamic bearing surfaces is compressed with increasing forceagainst the adjacent interior surface of the upper pump casing 1 withresult that a net axially downward pressure is exerted on the upperprojection surface of each raised impeller body. In operation, thethickness of the blood layer between the bearing surfaces 34 and theadjacent housing surface is a function of the fluid viscosity, theimpeller rotational speed and the geometry of the impeller bearing. Asthe fluid viscosity increases the fluid layer thickness increases. Asthe rotational speed increases the fluid layer thickness increases and,because of the net axial hydrodynamic pressure on the impeller and thefact that the impeller is suspended within the pumping chamber in partby a magnetic preload described below, the distance from each bearingsurface 34 to the adjacent upper casing face can change with rotationalspeed and fluid viscosity. However, in one embodiment that distance willbe within the range of from 0.003 inches to 0.020 inches.

Each raised impeller body 32 may also have wedge-shaped region forming apressure relief surface 38 downstream of the bearing surface 34. Thepressure relief surface 38 ensures a controlled and predictable loweringof the hydrodynamic pressure to minimize the blood shear stress andhemolysis. In addition, each pressure relief surface assists in defininga secondary flow path for blood within the pumping chamber whereby bloodexiting a bearing surface 34 is re-entrained across the adjacentpressure relief surface into the next downstream impeller flow slot orchannel 33, and from there into a lateral annular space defining thediffuser portion of the pumping chamber.

A relatively flat surface area on the upper surface of each impellerbody defines a substantially planar bridging surface 39 between eachexit end 37 of a bearing surface 34 and the associated pressure reliefsurface 38. In one embodiment, the width of each of the bridgingsurfaces 39 at its narrowest point is about 0.050 with a reasonabletolerance of ±0.028 inches. In such an embodiment, the pressure reliefsurface 38 may be inclined relative to the horizontal at an angle offrom 2 to 4 degrees.

Referring now to FIG. 3 there is shown in perspective one of thehydrodynamic bearing surfaces 34. Each bearing surface is ofapproximately uniform width from the entrance region 36, which defines ajunction edge 41 with the leading substantially vertical sidewall, forexample sidewall 33 a of the slot 33 (FIG. 2), to the exit region 37. Inone embodiment, the junction edge 41 is relatively sharp, having amaximum radius of curvature of less than 0.010 inches, and may be assmall as 0.005 inches or smaller. As indicated, each bearing surface 34is inclined upwardly from the entrance end 36 at an angle of less than 1degree relative to the horizontal and terminates at approximately theflat bridging surface 39.

In one embodiment, each bearing surface 34 is bounded along its lengthon opposite sides by inner and outer shrouds 43 and 44, respectively.The outside surface of the outer shroud defines a portion of theperipheral surface of the impeller. In operation, the inner shroud 43and the outer shroud 44 effectively minimize the fluid leaking out ofthe sides of the bearing surfaces thereby assisting the retention ofblood engaging the bearing surface to maximize the fluid layer thicknessand minimize the fluid shear stress. The shrouds also serve to guide theblood toward the exit end 37 of the bearing surface from which it flowsover the pressure relief surface 38 and into the next downstream flowslot 33. The top surface of each of the shrouds 43 and 44 is relativelyplanar or flat and, in one embodiment, each has a width of not less than0.020 inches. The top surface of each of the shrouds 43 and 44 may behigher than the entrance end 36 of the bearing surface 34 by about 0.230inches. At the exit end 37 of the bearing surface, the top surface ofthe shrouds 43 and 44 and the bearing surface may merge into the planarbridging surface 39.

In one embodiment, there is formed on each of the raised impeller bodies32 an inwardly facing and downwardly tapered curved section 46 inside ofthe inner shroud 43. The axial drop distance for each section 46 isabout 0.012 inches and the angle of taper is about 8°. The section 46assists in directing blood deflected from the conical surface 28 of thecentral post 24 to the central portion of the impeller, which then flowsfrom there into the slots 33 formed between the impeller bodies 32.

The inner surface of the upper pump casing 1 is provided with an upperannular ceramic disk (not shown) similar to the lower ceramic disc 27 onthe inner surface of the lower pump casing 2. The upper ceramic diskserves to minimize friction on start-up of the pump. An annular flange40 formed at the inner end of the inner cylindrical section 9 of thecannula 7 (FIG. 5) serves to retain the upper ceramic disc in place. Theceramic disks reduce electrical losses between the motor stators(described below) and the rotor magnets within the impeller, as well asprovide very flat surfaces for the hydrodynamic thrust bearings on theimpeller top surface. When the impeller is at rest it sits against thesurface of the upper ceramic disc. When rotational speed is imparted tothe impeller during startup, the impeller lifts off of the upper ceramicdisc and becomes fully suspended as described below. The impeller may becoated with titanium nitride to minimize wear during the starting andstopping process of the pump.

The impeller may be a single integral structure made of a magneticallyisotropic alloy. The material of a one-piece impeller of the typedescribed above may be biocompatible to avoid having to coat theimpeller or sub-assemblies. An example of a suitable magneticallyisotropic biocompatible material is an alloy of approximately 77.6%platinum (by weight) and 22.4% (by weight) cobalt. Such a one-pieceimpeller may be easier and less expensive to manufacture than impellersformed from multiple parts. Each raised impeller body 32 may have amagnetized portion. Magnetization of such an impeller may be performedby techniques known in the art, such as the exposure to a relativelystrong magnetic field. In one embodiment, the raised projection surfacesof each of the impeller bodies may be magnetized to provide magneticpoles. The magnetic poles of the impeller couple magnetically withmagnetic poles provided by motor stators 69 (FIG. 5) thereby enablingone or both of the stators to provide both a magnetic drive force tocause the impeller to rotate within the pumping chamber and magneticaxial and radial support. In one embodiment, every other upperprojection surface is magnetized to the same magnetic pole while theprojection surfaces therebetween are magnetized to have the oppositemagnetic pole. For example, where an upper projection surface has aNorth magnetic pole each projection surface on either side has a Southmagnetic pole. The particular arrangement of magnetic poles may bedetermined as desired without departing from the scope of the presentinvention. It will be understood that the motor stator coils that drivethe impeller provide magnetic poles in a pattern complementary to thoseemployed on the impeller.

Referring now to FIG. 4, there is shown in perspective the underside ofthe impeller 22 in which each raised impeller body 32 is hollowed out todefine a plurality of interior cavities or pockets 47. In cross section,each pocket 47 substantially corresponds in size and shape to the raisedimpeller body which defines its boundaries. The upper projection surfaceof each such raised impeller body contains a hydrodynamic bearingsurface which defines the top of the interior cavity therebeneath. Inone embodiment, the outer curved boundary of each of the pockets isconcentric with the impeller and subtends an angle relative to thecenter of the impeller of about 56.5 degrees. The inner radius of eachsuch pocket relative to the center of the impeller is about 0.4 inchesand the outer radius is about 0.665 inches. The pockets are locatedabout 90 degrees apart around the periphery of the impeller. Asdescribed in detail below, the pockets 47 are adapted to receive rotormagnets forming part of the motor drive system for the impeller. Thepockets 47 are separated by a plurality of substantially equally sizedinwardly projecting wall members 48 integrally formed with the impellerand defining substantially horizontal flat lower surfaces or shelves 49terminating radially inwardly at curved edge portions 51. In oneembodiment, there are four such wall members, each of which is situatedbetween two pockets. Each wall member and pocket is situateddiametrically opposite a corresponding wall member or pocket. The edgeportions 51 define a boundary of a substantially vertical inwardlyfacing curved surface 52, substantially concentric with thecircumference of the impeller.

A hollow cylinder 53 projects axially inwardly and defines the centralhole 23 of the impeller. In one embodiment, the central hole has adiameter of about 0.437 inches. When the pump is assembled, the centerpost 24 extends through the cylinder 53 into the pumping chamber. In oneembodiment, the radial gap between in the inner diameter of the cylinder53 and the outer diameter of the center post 24 is about 0.022 inches.

An annular cavity or space 54 is formed between the hollow cylinder 53and the curve surfaces 52. In this embodiment, the annular cavity 54 hasan inside diameter of about 0.437 inches, an outside diameter of about0.575 inches, and is adapted to receive passive magnetic bearingcomponents, as described in detail below.

Each of the wall members 48 may be provided with one or more balancingholes or bores 56 which are formed to ensure a balanced and evenrotation of the impeller during operation of the pump. In one embodimenteach wall member is provided with a set of two balancing holes ofunequal depth and approximately equal diameters situated side-by-sidealong a radius of the impeller. In this embodiment, the depth of thebalancing hole closest to the center of the impeller is about 0.10inches, while the depth of the outermost balancing hole is about 0.25inches. Each set of holes is situated diametrically opposite anotherset, whereby the diametric distance between the outermost holes of twoopposite sets of holes is about 1.22 inches and the diametric distancebetween the inner most holes of the sets is about 1.02 inches.

With reference to FIG. 5, there is shown a cross-sectional view of anassembled rotary blood pump according to an embodiment of the presentinvention. The upper casing 1 has affixed thereto the inflow cannula 7with its inlet channel 11. The outflow port 13 is formed by joinder ofthe half-round tubular extensions 14 and 16. The center post 24 extendsupwardly into the pumping chamber through the bottom of the lower casing2.

With reference to FIGS. 5 and 6, in one embodiment an impellersuspension system utilizes a passive magnetic bearing to provide radialimpeller support with respect to the center post 24. The passivemagnetic bearing is adjustable to provide an axially directed magneticpreload adapted to be resisted by the forces generated by thehydrodynamic thrust bearings described above in connection with each ofthe impeller blades 32. In one embodiment, one portion of the passivemagnetic bearing is formed by a stack 56 of permanent bearing magnets 57enclosed within the center post 24. The stack 56 may consist of threering-shaped permanent magnets 57 placed one on top of the other andcoaxially aligned along the axis of rotation of the pump impeller 22.Each of the ring magnets 57 has an axial height of less than 0.10inches, and an outer diameter of about 0.34 inches.

In one embodiment, and as seen best in FIG. 6, each of the three centerpost bearing magnets 57 may provide a magnetic vector oriented in theaxial direction, for example either north-on-top, south-on-bottom (N-S)or south-on-top, north-on-bottom (S-N). Thus, the stack of center postbearing magnets 57 may have alternating magnetization such that thepolarizations of the magnets within the stack may be N-S, S-N, N-S orS-N, N-S, S-N, as desired, whereby the magnetic forces established byeach ring shaped magnet 57 of the stack 56 act to repulse its adjacentmagnet in the axial direction.

As there are repulsive forces between each magnet, the magnets may befixed to or otherwise mechanically held in their coaxial relationship bysuitable engagement with an axially positioned center post rod 58. Toensure that the ring magnets are held in place, each magnet may beprovided with a thin ring-shaped spacer or washer 59 on the top and thebottom of the magnet, the upper most spacer being engaged beneath aprotruding circular flange 61 formed near the top of the center post rod58 to assist in holding the magnets in their coaxial arrangement. Thespacers 59 may also function to minimize demagnetization caused by theproximity of the stacked magnets. In one embodiment each such spacerwould have a thickness of less than 0.015 inches. Alternatively, wheredesired, the spacers may be adapted to act as flux concentrators forre-directing and concentrating in the radial direction the magnetic fluxproduced by the magnets 57. Alternative embodiments for the magneticvectors of the permanent magnets forming the stack 56 within the centralpost 24 may be employed without departing from the scope of the presentinvention. For example, the N-S orientations may be radial, with Northon the left and South on the right.

The other portion of the passive magnet bearing for the impeller isformed by another stack 62 of ring-shaped permanent magnets 63 placedwithin the impeller and surrounding the cylinder 53. The stack 62 mayconsist of three ring-shaped permanent magnets 63. As shown in FIG. 6,each impeller bearing magnet 63 has a magnetic vector oriented in theaxial direction with, for example, either north-on-top, south-on-bottom(N-S) or south-on-top, north-on-bottom (S-N). In one embodiment, themagnet pole arrangement of the stack of impeller magnets 63 correspondsto the magnetic pole arrangement of the stack of center post bearingmagnets 57. Thus if the stack of center post bearing magnets 57 has itsmagnetic vectors oriented N-S, S-N, N-S then the magnetic vectors of theadjacent stack 62 of impeller magnets 63 may also be N-S, S-N, N-S.Provided there is a sufficient radially oriented magnetic fluxconcentration, such an arrangement of magnetic vectors, and others,would effect repulsive forces between the corresponding stacks 56 and62, thereby establishing, in operation, a radially acting magneticbearing between the rotating impeller and its fixed center post. In oneembodiment, the inner and outer diameters of the ring-shaped magnets 63within the impeller are about 0.44 inches and 0.52 inches, respectively,while the radial distance between the ring-shaped magnets 63 within theimpeller and the ring-shaped magnets 57 within the center post 24 isabout 0.050 inches.

With reference to FIG. 5, in one embodiment, the axial alignment of thecenter post magnet stack 56 with respect to the impeller magnet stack 62may be adjustable so as to provide a selected axial preload force thatbiases the impeller toward the upper casing 1. In one embodiment, theflange 26 on the center post 24 holds the center post in positionrelative to the lower pump casing 2. The center post rod 58 extendsupwardly through the center post and is axially movable within thecenter post by an appropriate adjustment screw 66 which threadablyengages the lower end of the rod 58. Appropriate thread density could beon the order of 64 threads per inch.

The adjustment screw has a cap 67 engageable from beneath the impellerto adjust the axial position of the center post rod 58 and thereby thealignment of the impeller and center post bearing magnets. Thus, thecenter post rod 58 may be moved downwardly, for example, thereby movingthe center post magnet stack 56 downwardly relative to the impellermagnet stack 62, as shown in FIGS. 5 and 6. When the misalignmentbetween the corresponding magnet stacks 56 and 62 reaches approximatelythat shown in FIGS. 5 and 6, it will be apparent that repulsive forcesbetween the N-S, S-N, N-S magnetic vectors of the impeller stack and theN-S, S-N, N-S magnetic vectors of the center post stack will provide apreload axial force that biases the impeller toward the upper pumpcasing 1 and assists in keeping the impeller running near the innersurface of the upper casing. When the desired magnet alignment isestablished, the cap 67 may be welded to the center post to establish ahermetic seal and to prevent inadvertent movement of the adjustmentscrew. The adjustment screw is thereby sealed at the outside surface ofthe lower pump casing 2. Other mechanical arrangements suitable foradjusting the axial position of the stack 56 may be adopted withoutdeparting from the scope of the invention.

When the pump is activated, the axial upwardly directed magnetic preloadforce caused by the offset between corresponding stacks of bearingmagnets is balanced against the downward force in the axial directioncreated by the hydrodynamic thrust bearings on the impeller uppersurface. Therefore, the impeller may be suspended in both the axial andradial directions and is submerged within the blood filling the pumpingchamber. The inner and outer magnet bearing assemblies 56 and 62 thuswork together to provide primary radial and axial stiffness to avoidwear and to ensure the presence of yet another open flow path for theblood being moved through the pump. This flow path is from the housingwhere the fluid collects after exiting the impeller flow slots 33,underneath the impeller and up through the annular gap between thecenter post and the impeller that is maintained by the passive magneticbearing described above, from where the blood is re-entrained throughthe impeller flow slots 33 into the primary flow path described above.The impeller hydrodynamic thrust bearings described above provide axialstiffness only when the impeller is running near the inner surface ofthe upper casing 1.

As indicated above, the pump of the present invention may include athree-phase dual stator axial flux gap motor for driving the impeller.An advantage of a dual stator motor is that one of the stators may beused to cause the impeller to rotate should the other stator fail tofunction. In one embodiment, the lower stator is spaced farther from theimpeller 22 than the upper stator so as not to degrade a net axialpreloading of the impeller resulting from its magnetic interaction withthe upper stator. With reference to FIGS. 5 and 7, the impeller isprovided with a set of four drive magnets 68. Each drive magnet 68 iscontained within one of the pockets or cavities 47 (FIG. 4) formedwithin the raised portions 32 of the impeller at the underside of theimpeller. The drive magnets 68 are enclosed within the impeller by asuitable annular base plate 70.

As shown in FIG. 5, one stator is located above the impeller on theupper pump casing 1 while the other stator is positioned below theimpeller on the lower pump casing 2. Each stator contains a plurality ofthe motor drive windings 69 and a back iron ring 71. The motor drivewindings 69 consist of coils of electrically conductive wire and may becircular in cross section or have other appropriate cross-sectionalconfigurations, as desired. In one embodiment shown in FIG. 8, the coilsare circular in cross section and each stator consists of six such coilsplaced on the outside of the respective back ring. The coils are placedon the back ring such that the coil axis is perpendicular to the surfaceof the ring. As will be understood by those skilled in the art, themotor drive coils 69 generate electromagnetic fields that interact withthe magnetic fields of the impeller drive magnets 68 to cause theimpeller to rotate. The back iron ring 71 serves to enhance the magneticflux produced by the drive magnets. The magnetic forces produced by themotor stator coils also provide secondary radial impeller and axialmagnetic preloading support to the impeller. The result is that theimpeller is dynamically balanced in both the radial and axial directionsduring normal operation. It will be understood that only a single statoris needed to operate the pump motor of the present invention. Two statorassemblies are desirable because if one stator assembly should fail, theother will operate the motor, although operating power consumption willbe increased.

Each stator is contained within a stator can 72, 73. Each stator can ishermetically sealed to its respective pump casing and, in oneembodiment, has a thin wall less than 0.007 inches thick closest to themotor drive magnets 68. The thin wall allows the use of the ceramicdiscs between the impeller and the stators. Each stator can has ahermetic feed-through arrangement for the electrical connections to theconjoined external headers or connectors 29 and 31.

FIG. 9 illustrates an implanted rotary blood pump according to anembodiment of the present disclosure. The inflow cannula 7 is insertedapically into the left ventricle 74 of the patient's heart 76. A bloodtransport graft or tube 17 connects the blood outlet of the rotary bloodpump to the patient's aorta 18. The power and control cable 77 may beconnected to a controller 78 having a power source 79. The controller 78and the power source 79 may be implanted within the patient's body orworn by the patient. The controller is used to provide cliniciansinformation on how the device is performing, to provide run status andalarm conditions and controls the rotational speed of the impeller, asmay be desired. For example, impeller rotational speed may be controlledby using a pulsed drive waveform and measuring the back emf of the rotorwhen the drive pulse is at zero. Such a technique is set forth incommonly owned International Application No. PCT/US00/40325 having anInternational Publication Number WO 01/05023 A1, incorporated herein byreference.

The above specific embodiments are illustrative, and many variations canbe introduced on these embodiments without departing from the spirit ofthe disclosure or from the scope of the appended claims. For example,elements and/or features of different illustrative embodiments may becombined with each other and/or substituted for each other within thescope of this disclosure and appended claims.

1. A rotary blood pump comprising: a housing defining a cylindricalpumping chamber having a blood inlet and a tangential blood outlet; arotatable impeller within said pumping chamber, said impeller having arotational axis and a plurality of raised sectors surrounding saidrotational axis, each of said raised sectors having an upper surfacearea defining a tapered region curved in the direction of rotation ofsaid impeller and having a leading portion and a trailing portion suchthat a gap between each of said tapered regions and an interior surfaceof said housing decreases from said leading portion to said trailingportion to create a hydrodynamic thrust bearing with said interiorsurface of said housing when said impeller is rotating; a fluid flowpath between each pair of adjacent sectors, each of said fluid flowpaths having parallel sidewalls perpendicular to the sidewalls ofadjacent flow paths and extending along an inclined bottom surfacedescending across the axial height of said impeller to a radiallyperipheral exit at the base of said impeller, the surface area of aleading sidewall of each flow path being less than the surface area ofthe opposite sidewall of said flow path; and inner and outer boundariesof each of said tapered regions being defined by inner and outercoextensive shrouds, an interior side of each of said inner shroudsdefining an inwardly facing and concavely shaped surface areacoextensive with said inner shroud and descending toward said rotationalaxis of said impeller.
 2. The rotary blood pump of claim 1 comprising atleast four of said raised sectors.
 3. The rotary blood pump of claim 1in which the leading portion of each of said tapered regions comprisesan entrance edge along the top of a sidewall of one of said fluid flowpaths.
 4. The rotary blood pump of claim 3 in which said top surface ofeach of said inner and outer shrouds is higher than said entrance edgeand substantially flat.
 5. The rotary blood pump of claim 4 in whichsaid top surface of each of said shrouds is higher than said entranceedge by approximately 0.230 inches.
 6. The rotary blood pump of claim 5in which each of said shrouds is approximately 0.020 inches wide.
 7. Therotary blood pump of claim 3 in which an inner region of each of saidfluid flow paths extends inwardly beyond a respective sidewall in adirection substantially tangential to said rotational axis.
 8. Therotary blood pump of claim 1 in which the outer surface of each of saidouter shrouds defines a portion of the periphery of said impeller. 9.The rotary blood pump of claim 1 in which said impeller contains analloy of approximately 77.6% platinum by weight and 22.4% cobalt byweight.
 10. The rotary blood pump of claim 1 in which each of saidconcavely tapered surface areas terminates at a vertical wall portionextending to and merging into one of said trailing sidewalls of one ofsaid flow paths.